Synthesis, Characterization And Catalytic Property Of Polyoxometalate Clusters Decorated By Transition Metal-biimidazole Complexes

Six molybdophosphate POM inorganic-organic hybrids were obtained under hydrothermal conditions or routine synthetic method by self-assembly of simple oxyacid (or Oxysalt) and2,2’-biimidazole. Compounds1-5are Strandberg-type POMs, and compound6is Keggin-type POM. Transition metal ions (ZnⅡ、NiⅡ、MnⅡ) and2,2’-biimidazole (H2biim or biim2-) were introduced into the skeletons of these polyoxoanions. These compounds were characterized by single-crystal X-ray diffraction, elemental analysis, IR spectroscopy, TG analysis and electrochemical analysis. Their electrocatalytic properties toward the reduction of H2O2were investigated by cyclic voltammetry (CV), and the catalytic activites for the oxidation of cyclohexanol to cyclohexanone and the synthesis of cyclohexanone ethylene ketal were studied.1. Three novel Strandberg-type POMs mono-supported by Zn-2,2’-biimidazole subunits were obtained under routine conditions. The2,2’-biimidazole ligands coordinated with Zn(Ⅱ) as mono-, bi-and tetradentate ligands. Especially in compound1, two Zn(Ⅱ)-{P2Mo5} cluters are bridged by tetradentate ligand μ-biim2-, this coordination type of2,2’-biimidazole was first found in POMs. Most importantly, compounds1-3represent the first example where the Strandberg-type POMs are used as acid-catalysts in an organic reaction, and they have better catalytic activity than their parent.{H9(H2biim)7}[μa-biim){(Zn(H20)2)0.5(HP2Mo5O23)}2]·7H2O (1){H6(H2biim)3}[Zn(H2biim)(H3biim)(H2O)(P2Mo5O23)]2·3H2O (2){H10(H2biim)7}[Zn(H2biim)(H2O)2(P2Mo5O23)][P2Mo5O23]·9H2O (3)2. A charge-transfer salt containing two [(HPO4)MosO15]4-polyoxoanions and four [Ni(H2biim)3]2+coordinate cation was synthesized under hydrothermal conditions. In this compound, Ni-H2biim complexes were introducd:[Ni(H2biim)3]4[(HPO4)2Mo5O15]2·3H2O (4)3. Using a routine synthetic method, a Strandberg-type POM with a symmetric dumbbell-like structure bridged by a [Mn(H2O)4]2+cation was self-assembled in aqueous solution:{H8(H2biim)5}{Mn(H2O)6}[μ-Mn(H2O)4{(HPO4)2Mo5O15}2]·14H2O (5)4. A novel Mn-H2biim-{PMo12} coordination polymer with Keggin skeleton was synthesized under routine conditions by self-assembly in aqueous solution. A1D infinite chain and Mn-Mn bond in this compound:{[Mn7(H2biim)7{PMo12O40}2]-16H20}n (6)...